1. Field of the Invention
The present invention relates to a process for preparing a thin transparent resin film having a high light transmittance which is suitable as a dust cover of a photomask.
2. Description of the Prior Art
In the production of integrated circuits, a projection printing method is employed which comprises projecting a pattern drawn over a photomask on a silicon wafer coated with a resist by using light to effect light deterioration or light hardening of the resist in the portions corresponding to the pattern. When some undesirable adherent matter, i.e. dust, is present on the photomask, it is projected on the wafer. In order to avoid such an influence of the dust, a method for using a dust cover of a thin resin film is known in U.S. Pat. No. 4,131,363. The cover serves to improve the yield of integrated circuit chips produced and reduces the number of times required to clean a photomask. That way the service life extends.
The projection printing method now includes new modes, which involves respective determined thicknesses of thin films suitable as the dust covers for the respective modes. More specifically, a thin film having a thickness of 2.86.+-.0.2 .mu.m in the case of a projection mode (light exposure with equal magnification) and 0.87.+-.0.02 .mu.m in the case of a stepper mode (light exposure with reduced magnification). This condition of thickness is a requisite for securing a light transmittance of 96% or more in the thin film in the case of g line (436 .mu.m) as the light source used for light exposure. The reason will now be explained in conjunction with the accompanying drawings.
In FIG. 1, the solid line curve (a) shows light transmittance of a thin cellulose nitrate film having a thickness of 2.90 .mu.m, which is plotted against the wavelength, the light transmittance curve has a wavy form having peaks showing a light transmittance of approximately 100% and bottoms showing a light transmittance of only about 80%. Part of the light is reflected at the interface between the film and air to go backward, and part of the reflected light is reflected from the interface between the film and air on the reverse side to advance forward. The advancing light after reflecting twice is delayed, in advancement, by twice the film thickness as compared with directly transmitted light. Thus the two advancing lights have the same wavelengths but provide a phase difference to cause mutual interference leading to loss of light transmittance. When the film thickness is 2.86.+-.0.2 .mu.m, the g radiation transmittance appears around the the peak. However, this cannot be attained if the film thickness varies, since a change in the film thickness causes a shift of the light transmittance curve rightward or leftward. FIG. 1 also shows light transmittance of a thin cellulose nitrate film having a thickness of 0.90 micron with a dot-dash line (b).
Even if the influence of dust can be obviated by providing a dust cover, the amount of light for effecting projection must not be reduced by the dust cover. U.S. Pat. No. 4,131,363 also discloses an instance of a light exposure apparatus including a plurality of dust covers, each of which preferably has a light transmittance of 96% or more. Such covers must be prepared from a highly transparent, non-oriented material of thin film with a high precision of a predetermined and constant film thickness. Since the thin film has an extremely small thickness of 10 .mu.m or less, it must be held by a support frame in a uniformly tensional state thereof.
Japanese Patent Laid-Open No. 219023/1983 discloses a method of preparing such a thin film from cellulose nitrate and a method of holding such a thin film. More particularly, it discloses a process for preparing a thin, non-oriented cellulose ester film kept in a uniformly tensional state, characterized by comprising the step of casting a solution of a cellulose ester dissolved in an organic solvent over a smooth glass plate and removing the solvent to form a thin film having a uniform thickness on the glass plate, the step of separating the thin film from the glass plate in water, and the step of recovering the thin film from the water, holding the thin film in a wet state on a support frame, and drying the thin film. Thus, the so-called casting method is employed in this process to obtain the non-oriented film. Since the cellulose ester is easily dissolved in a relatively low boiling point solvent such as a ketone or an ester of lower aliphatic acid, a thin film having a predetermined final thickness can be prepared by controlling the concentration of a solution and the casting thickness. A film having a constant thickness of 2.8.+-.0.3 .mu.m, 4.5.+-.0.3 .mu.m, or the like is used as the thin photomask-protecting film.
When a smooth glass plate used in casting, as the casting substrate is immersed in water after removal of the solvent, a thin film is spontaneously stripped off, so that it can be easily recovered. When the thin film thus recovered is directly mounted on a support frame, and coated, in a damp-dry state, with a small amount of a volatile solvent along the portion of the film in contact with the support frame, followed by drying, a uniformly tensional supporting state can be secured due to the slight shrinkage of the film as well as the adhesion of the film to the support frame.
With recent progress in the semiconductor industry, there is a trend toward increases in the density and scale of integration of integrated circuits. Accordingly, the line width and interlinear distance of a projected pattern on a wafer tend to be decreased.
As a result, h line (406 .mu.m) or i line (365 .mu.m) having a shorter wavelength and a higher energy than the g line has often been used as the exposure light source. Resists capable of being rendered photosensitive by respective lights alone have also been increasingly employed properly. Thus, if conventional types of thin films as the dust covers are to be used, those having a variety of film thicknesses adapted to respective light sources must be prepared and properly used according to objectives.